In recent years, a glass substrate has been used as one of magnetic disk substrates suitable for increasing the recording density. The glass substrate has a higher rigidity than a metal substrate and thus is suitable for an increase in rotational speed of a magnetic disk device. Further, since the glass substrate can obtain a smooth and flat surface, it can reduce the flying height of a magnetic head and thus is suitable for improving the S/N ratio and increasing the recording density.
In general, a glass substrate for a magnetic disk is manufactured by sequentially carrying out a process of heating and melting a glass material to prepare a molten glass, a process of forming this molten glass into a plate-shaped glass disk, and a process of processing and polishing the glass disk formed into the plate shape to thereby manufacture a glass substrate.
For forming the molten glass into the plate-shaped glass disk, a forming method such as a press method or a float method is employed. In the press method, a cylindrical glass base member is cut into a glass disk having a thickness slightly greater than that of a magnetic disk substrate and then the glass disk is adjusted in shape and processed to a required flatness and thickness. In a grinding process of processing the glass disk to the required flatness and thickness, the glass disk is ground to the target thickness and processed to the target flatness using a lapping machine. The processing is carried out by holding the glass disk between upper and lower surface plates of the lapping machine and rotating them in opposite directions while using free abrasive particles (slurry). As the free abrasive particles, use is made of abrasive particles with a particle size according to a predetermined dimensional accuracy and shape accuracy of the glass disk. In first grinding (lapping 1), the grinding is carried out using free abrasive particles with a large particle size (e.g. average particle size: about 30 μm), while, in second grinding (lapping 2: precision lapping process), the grinding is carried out using free abrasive particles with a small particle size (e.g. average particle size: about 10 μm).
In the float method, a glass disk is cut out from a plate-shaped glass manufactured by the float method and is adjusted in shape and its main surfaces are ground. The plate-shaped glass manufactured by the float method has mirror-finished main surfaces from the beginning and thus is excellent in flatness and thickness as compared with the pressed glass. Therefore, the lapping 1 using the free abrasive particles with the large particle size is omitted and the grinding (lapping 2) using the free abrasive particles with the small particle size is carried out from the beginning.
As a prior art relating to this, JP-A-2002-55061, for example, is cited.